BIHANG TILT. K. SV. VET.-AKAD. IIANDL. BAND 13. AFD. IV. N:0 5. 83 



Like the other Arbaciadae tliis genus is heteropodous, the 

 (lorsal pedicels being, presumably, branchial, the ventral loco- 

 motive. Accordingly the difference in shape is conspicnous 

 between the dorsal and the ventral peripodia. From the ra- 

 dial of the calyeinal system to the ambitus each zone of pores 

 is simple, narrow, slightly imdulating, a single series of tians- 

 verse, depressed peripodia, with large perforations and the 

 separating bridge narrovvf and low. Close under the ambitus 

 all this is changed. The peripodia decrease in breadth and 

 become more oval, at last nearly circular, the bridge swells 

 and encroaches upon the perforations, the margin is flattened, 

 and the locomotive form soon comes out, with the diagonal 

 gradually directed adorally, w^hile the simple arrangement is 

 exchanged for rows of three, at first nearly longitudinal, then 

 gradually approaching to the transverse, thus causing the zone 

 to expand largely. Round the peristome the peripodia are 

 disposed conformably to the general rule^), thus: 2, 3, 3 in 

 the series I a . . . V6, 2, 2, 3 in the series Ib . . . Va. 



The spine-bearing tubercles present the remarkable cha- 

 racter of being all primary, none secondary. 



On the ambulacra they alternate in two vertical approxi- 

 mate series bordering on each sidc upon the zone of pores. 

 Troschel drew attention to a marked difference in their dis- 

 position. Below, and visible from the second or third plate 

 from the peristome, the two series are in all the species con- 

 tiguous, and in some instances, as in the well-known A. aequi- 

 tuberculata Blv., contiuue so all along, while in others they 

 separate a little near the ambitus, and thiis, as in the A. lixula 

 L., leave between them a narrow areola pointed below and 

 abovc. It is however only in full-grown or nearly full-grown 

 specimens that this character is fully developed and made 

 more apparent; young specimens have the two ambulacral 

 series more closely approximated. In the upper part of the 

 ambulacra the two series become, in all the species, more or 

 less deranged from the unequal development of the tubercles 

 and their more or less sudden diminution or even partial de- 

 ficiency, so as to end apparently with a single series. 



On the interradia the tubercles are disposed vertically on 

 each half in from three to six series, and transversely in re- 



') Etudes s. 1. Ecbinoidées, p. 25, 26, t. XVIII, f. 156. 



